Conventionally, as a film superior in cut property, cellophane is known. Cellophane has been conveniently used for various packaging materials and adhesive tapes in view of its properties such as superior transparency, easy cut property, twist property and the like. On the other hand, however, cellophane shows different properties for each season since it has hygroscopicity, and stable supply of cellophane having a constant quality is difficult. In addition, packaging bags, adhesive tapes and the like using polyethylene terephthalate as a base film have been used for their superior properties of stretched polyethylene terephthalate film such as toughness, heat resistance, water resistance, transparency and the like. Despite these superior properties, however, they are defective in that cutting is difficult, and therefore, packaging bag cannot be easily torn open, and adhesive tapes are difficult to cut, as a result of which they cannot be used for packaging and the like.
As a method to solve the above-mentioned problems, a monoaxially-oriented polyester film laminated on paper, a biaxially-oriented polyester film obtained by copolymerizing a large amount of diethylene glycol component and the like, a biaxially-oriented polyester film obtained by admixing a polyester resin having a low molecular weight, a laminate polyester film obtained by laminating, on at least one surface of polyester resin layer (A), a polyester resin mixture layer (B) having a melting point higher by not less than 10° C. than the melting point of polyester resin layer (A), which layer (B) is made of a of a terephthalic acid crystalline polyester resin (a), and a crystal segment and having a melting point of not less than 170° C. and a melting point or softening point of not more than 100° C., and the like have been proposed (see patent references 1-4).
patent reference 1: JP-B-55-8551
patent reference 2: JP-B-56-50692
patent reference 3: JP-B-55-20514
patent reference 4: JP-A-2002-337290
However, a plastic film obtained by molecular orientation in a monoaxially direction in the above-mentioned prior art is defective in that it can be linearly cut easily in the orientation direction but is difficult to cut in the direction other than the orientation direction, and a polyester film obtained by copolymerization of a large amount of a diethylene glycol component and the like is defective in that copolymerization impairs the inherent property of polyethylene terephthalate. In addition, a polyester film obtained by admixing a low molecular weight polyester resin allows easy development of troubles such as film breakage in a stretching step and is not practical.
Furthermore, a polyester film obtained using a mixture of a terephthalic acid crystalline polyester resin and a polyester block copolymer as a resin forming a surface layer has a sea-island structure wherein the polyester block copolymer is dispersed in the crystalline polyester resin. As a result, while it shows easy-to-tear property, detachment easily occurs in the interface between the crystalline polyester resin and the polyester block copolymer, and therefore, during laminate processing, interlayer cohesion becomes inferior due to the adhesion between layers, an interlayer cohesive failure occurs, and as a result, laminate strength may be degraded sometimes.